Soldermask opening to prevent delamination

ABSTRACT

A multilayer circuit board includes a base layer, a conductive layer and a soldermask. The soldermask layer has two sets of openings. One of the openings are vent openings, that expose the base layer to provide ventilation so that gases may escape during processing. The second openings expose selective regions of a conductor layer. The multi-layer circuit board provides for less occurrences of delamination.

This application is a Continuation of U.S. application Ser. No.10/721,584, filed Mar. 25, 2003, which is a Divisional of U.S.application Ser. No. 10/095,329, filed Mar. 11, 2002, now U.S. Pat. No.6,671,182, which is a Continuation of U.S. application Ser. No.09/417,491, filed Oct. 13, 1999, now U.S. Pat. No. 6,356,452, both ofwhich are incorporated herewith.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to printed circuit boardsproduction and, in particular, to the processes utilized to createprinted circuit boards.

BACKGROUND OF THE INVENTION

A typical printed circuit board is a flat board that provides supportand electrical interconnection between microchips and other electroniccomponents. The base of the board can be made of reinforced fiberglassor plastic, and the electronic components are elezctricallyinterconnected by conductive pathways. There are several ways to makeprinted circuit boards. One method entails bonding a conductive foil,such as copper, over the base. A conductive pattern is then formed inthe conductor. One method of patterning the metal layer uses a negativeimage of the desired circuit pattern and a photo resist layer. The photoresist is activated using the image such that selected areas of thephoto resist can be removed. An etch process is then performed to removethe photo resist that was not activated and the underlying metal layer,leaving behind the conductive pathway pattern.

Today, most printed circuit boards are composed of several sheets orlayers. A multi-layer printed circuit board may be fabricated fromseveral composite sheets, each comprising a substrate of insulatingmaterial and a layer of metal, such as copper, attached to one surfaceof the substrate using a resin. A desired conductive pathway pattern isthen provided in the metal layer, as explained, and multiple layers ofinsulating material and metal conductor layers can be fabricated. Asoldermask layer can be provided over the top level of conductor tocontrol areas exposed to a soldering process. A finished printed circuitboard can then put through an assembly process where it is populatedwith various electrical components.

Delamination, or separation, problems have been discovered in theprinted circuit board assembly industry regarding processes similar tothe one described above. These problems are a result of the variousenvironmental stresses inherent to assembly procedures. For example,delamination of the soldermask layer can occur when moisture retained bya board's constituent components are exposed to post-production assemblyprocesses performed at high temperatures.

For the reasons stated above, and for other reasons stated below whichwill become apparent to those skilled in the art upon reading andunderstanding the present specification, there is a need for amulti-layer printed circuit board assembly that reduces the likelihoodof delamination.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of one embodiment of a simplified circuit boardaccording to the present invention;

FIG. 2 is a cross section view of the circuit board shown in FIG. 1;

FIG. 3 is a top view of an embodiment of a detailed circuit boardaccording to the present invention; and

FIG. 4 is a cross section view of the circuit board shown in FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description of the invention, reference ismade to the accompanying drawings which form a part hereof, and in whichis shown, by way of illustration, specific embodiments in which theinvention may be practiced. In the drawings, like numerals describesubstantially similar components throughout the several views. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention. Other embodiments may be utilizedand structural, logical, and electrical changes may be made withoutdeparting from the scope of the present invention. The followingdetailed description is, therefore, not to be taken in a limiting sense,and the scope of the present invention is defined only by the appendedclaims, along with the full scope of equivalents to which such claimsare entitled.

Printed circuit boards, as described above, can be fabricated usingmultiple layers of different materials. These materials can beinsulators, conductors or other materials used to provide benefitsduring processing such as soldermasks. Because these materials differ incomposition, they behave differently to environmental conditions such astemperature and humidity. Of particular interest to the presentinvention is the retention of water, or other liquids, by insulatingmaterial used in fabrication of circuit boards.

Soldermasks, as known to those skilled in the art, are used to maskareas of a conductor to prevent solder from coating the conductor. Thatis, some areas of the conductor may be required to be coated in solder,while other areas need to be protected from solder. By controlling thelocation of solder, conductive shorts can be prevented.

The soldermask layer can be screen printed over the conductor. Thesoldermask includes openings to expose selected areas of the conductor.It is desired that the soldermask remain adhered during processing. Ifthe soldermask layer delaminates from the conductor, conductive shortsor leakage paths can be formed.

The present invention provides a technique for reducing delamination ofa soldermask layer of a circuit board which results from out-gassing oflayers of the circuit board. FIG. 1 is a top view of a simplifiedcircuit board 100 according to the present invention. The circuit boardincludes a conductor trace 110 and a soldermask used to shield theconductive layer from potential shorts by exposing only selected areasof the conductor via conductor openings 130. The soldermask alsoincludes ventilation openings 140 to allow underlying layers of thecircuit board to vent gases during processing without producing adverseside effects to the adhesion of the soldernask. The details of the ventopenings, or holes, are described in greater detail below.

FIG. 2 illustrates a cross-section view of the circuit board of FIG. 1.The soldermask 120, or solder resist, allows access to the conductor 110via conductor openings 130. Unlike prior soldermasks, the presentinvention provides an opening, vent holes 140 to expose a base layer150. These openings allow for ventilation of the base layer. The baselayer can be comprised of any electrically insulating material. In oneembodiment, the base material is Mitsubishi HCL832. The base layer canbe fabricated using multiple layers, and is not limited to a singlematerial. The soldermask can be either a sheet of insulating materialwhich is adhered to the top of the base layer and conductor or aconformal coating. In one embodiment, the soldermask is a sheet of TaiyoAUS5 which is adhered to the board using a screen printing operation.Likewise, the conductor traces can be any suitable conductive, orsemi-conductive material such as copper.

As explained herein, the ventilation holes are provided in thesoldermask layer to allow the base layer to out-gas. As such, the ventholes are located in a region of the circuit board which do not containconductors. Because the soldermask in one embodiment is a sheet ofmaterial, the ventilation holes are patterned into a plurality of holes.That is, an array of holes is preferred to providing one large hole. Thelocation and number of the ventilation holes will be dependant upon thedesign of the circuit board. In general, maximizing the cumulativesurface area of ventilation holes in the locations not containingconductors is desired.

FIG. 3 provides a top view of another embodiment of a circuit board 300according to the present invention, and FIG. 4 provides across-sectional view of the circuit board of FIG. 3. The circuit boardincludes a conductive grid array 360 for providing an interconnect forcircuit components, such as but not limited to resistors, capacitors,inductors, and integrated circuits. The soldermask layer 320 includesfirst openings to provide access to the grid array conductors, andsecond opening 340 to expose the base layer. These openings aregenerally provided along outside edges of the circuits board.

An integrated circuit die assembly is fabricated using the circuit board300, as illustrated in FIG. 4. The die assembly is comprised of thecircuit board base layer 450 with a soldermask layer 320 concealing aconductive layer 410. A rectangular shaped opening, or aperture 310 isprovided in the board. The opening passes through the soldermask layer,the conductive layer and the base layer. This opening provided access toan integrated circuit die, as explained below. A plurality of bond padsare located in regions 330 of the circuit board outside the aperture. Aplurality of solder bump conductor openings 360 are located on the sameside of the circuit board as the bond pads.

The die assembly comprises the integrated circuit die (IC) 460 which hasits top side affixed to the bottom side of the circuit board. The IC canbe affixed to the board with an adhesive 470. The adhesive is applied tothe IC's top side such bond connections provided on it's top side areexposed and fully accessible through aperture 310 when attached to thebase layer. The IC can then be wire bonded through the opening to thebond pads provided in locations 330. The IC die and wire bonds are thenencapsulated with a non-conductive encapsulant 480 to provide anenvironmental seal.

Again, the soldermask layer included ventilation openings 340 to allowthe underlying layers to expel gases without adversely affecting theadherence of the soldermask layer. This is of particular interest whenmoisture retained by a board's constituent components (e.g. a soldermasklayer or a base layer) are exposed to high temperatures delamination mayoccur in regions of the circuit board.

CONCLUSION

An embodiment of the invention allows gases trapped between layers of amulti-layered printed circuit board to escape, in an effort to preventdelamination. A circuit board has been described which includes openingsin a particular layer(s) of the multi-level printed circuit board. Thepresent invention describes an improved multi-layer circuit board thatis more resistant to delamination problems. An embodiment of amulti-layer circuit board according to the present invention comprises abase layer, a conductive layer and a soldernask. The soldermask layerhas two primary sets of openings. One of these sets are the ventopenings, or vent holes, that expose the base layer to provideventilation so that gases may escape. The circumference, location anddepth of the holes are dependent upon the nature of the application.This invention allows a plurality of layers, such as the soldermasklayer, to be used in printed circuit board applications, whilepreventing delamination throughout the board's surface. The reduction indelamination offers the added benefit of allowing for improved handlingof the circuit board, particularly during various processing stages.

The above-mentioned problems with circuit board assemblies and otherproblems are addressed by the present invention and which will beunderstood by reading and studying the specification.

In one embodiment, a circuit board comprises a base layer, and aconductive layer located on a first side of the base layer. Theconductive layer is patterned into conductive traces. A soldermask layeris located on a surface of the conductive layer and a region of thefirst side of the base layer which is not covered by the conductivelayer. The soldermask layer is provided with a first plurality ofopenings to expose the base layer to provide ventilation, and whereinthe soldermask layer is further provided with a second plurality ofopenings to expose the conductive traces.

In another embodiment, a circuit board assembly comprises a base layer,and a conductive layer located on top of the base layer. The conductivelayer is patterned to form conductive traces. A soldermask is attachedover the conductor layer and the base layer. The soldermask layerincludes a first plurality of openings to expose a top surface of thebase layer and a second plurality of openings to expose areas of theconductors. An integrated circuit is also attached to a first side ofthe circuit board.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement which is calculated to achieve the same purpose maybe substituted for the specific embodiment shown. This application isintended to cover any adaptations or variations of the presentinvention. For example, by creating an avenue for gases to escape, thesoldermask will remain attached at the board's perimeter, leaving a flatsurface to clamp onto at the end of the assembly process. This makesfurther processing to the printed circuit board possible.

1. An assembly, comprising; a circuit board comprising a non-conductivebase layer and a conductive layer located on top of the non-conductivebase layer, wherein the conductive layer is patterned into traces; asoldermask comprising an insulating layer attached to the non-conductivebase layer and covering the conductive traces, wherein the soldermaskincludes a first plurality of openings selectively exposing theconductive layer and a second plurality of openings exposing thenon-conductive base layer; and the second plurality of openings includesan array of openings arranged to expose the maximum cumulative surfacearea of non-conducting base layer in areas not exposing the conductivelayer.
 2. The assembly of claim 1, wherein the circuit board includesmultiple layers of conducting and non-conducting layers.
 3. The assemblyof claim 1, wherein the first plurality of openings are circular inshape.
 4. The assembly of claim 1, wherein the first plurality ofopenings are rectangular in shape.
 5. The assembly of claim 1, whereinthe second plurality of openings are rectangular in shape.
 6. Theassembly of claim 5, wherein the rectangular openings include multipleheights and widths.
 7. The assembly of claim 6, wherein the rectangularopenings which are substantially vertically and substantiallyhorizontally aligned are the same size.
 8. The assembly of claim 1,wherein the second plurality of openings are circular in shape.
 9. Theassembly of claim 8, wherein the array of circular openings includemultiple diameters.
 10. The assembly of claim 9, wherein the array ofcircular openings which are substantially vertically and substantiallyhorizontally aligned are a single sized diameter.
 11. The assembly ofclaim 1, wherein the soldermask is a polymer-based solder resist.
 12. Anassembly including a soldermask, wherein the soldermask comprises: aprotective layer located on a first side of a circuit board attached toa non-conductive base layer and covering a conductive layer; a firstplurality of openings in the protective layer arranged to expose theconductive layer; a second plurality of openings arranged to expose thenon-conductive base layer; wherein the conductive layer is patterned toform traces; and wherein the second plurality of openings consist of anarray of substantially rectangular openings arranged to expose a maximumcumulative surface area of the non-conducting base layer in areas notcontaining conductive traces.
 13. The assembly of claim 12, wherein thefirst plurality of openings in the soldermask consist of an array ofsubstantially circular openings.
 14. The assembly of claim 13, whereinthe openings are substantially vertically and substantially horizontallyaligned.
 15. The assembly of claim 12, wherein the first plurality ofopenings in the soldermask include an array of substantially rectangularopenings.
 16. The assembly of claim 15, wherein the openings aresubstantially vertically and substantially horizontally aligned.
 17. Theassembly of claim 12, wherein the protective layer is a polymer-basedsolder resist.
 18. The assembly of claim 12, wherein the secondplurality of openings include an array with substantially horizontallyaligned rows and substantially vertically aligned columns.
 19. Theassembly of claim 18, wherein the rows and columns of the array may beincomplete.
 20. The assembly of claim 12, wherein the second pluralityof openings includes an array with a plurality of opening sizes.